The conventional way today to regulate the flow rate in tubes for infusion- and transfusion solutions is by means of clamping the tube in a so called roller clamp. It is however very difficult to provide an accurate adjustment of the flow area in the tube especially at low flow rates in this way. Besides the plastic material in the tube is exerted to high stresses at high degrees of compression and so called cold flow will easily occur. After this the ability of the tube material to revert its shape is detoriated. This is of course very unsatisfactory for infusion- and transfusion assemblies in medical treatment, where it is important that the drop rate can be set and adjusted with a high accuracy.
In SU-706-634 and SU-943-457 are shown valves for resilient cylinders for use in machine buildings and which valves are controlled by twisting the cylinders. On the inside of the cylinder longitudinal strips are arranged, which are intended to reduce the aerodynamic noise or which are filled with a thermo-sensitive material. These strips are however not designed to provide any substantial reduction of the compression of the cylinder when this is twisted. In order to provide a complete shut off of the flow a considerable twisting angle is required, which exerts the material in the cylinder to considerable stresses, and besides it is difficult to obtain an accurate adjustment of the flow area especially at low flow rate.